Add additional configuration options for client-side routing announcements

[madalynrose]

Summary

Add additional configuration options for the RouteAnnouncer created in #19290 by adding a <RouteAnnouncement> Gatsby component.

Basic example

<RouteAnnouncement visuallyHidden={true}>
  {blogPost.title}
</RouteAnnouncement>

or

<RouteAnnouncement>
	<h1>Blog Post Title</h1>
</RouteAnnouncement>

Motivation

Allowing developers to specify what exactly should be announced instead of inferring that information based on the h1 or title of a page will ensure route change announcements are accurate. This could be achieved by creating a Gatsby RouteAnnouncement component that can wrap relevant content, as proposed in this comment. This component could also have error checking to make sure announcements aren't too long and have an option for being visually hidden, since they are intended to be consumed by assistive technologies.